1. Field of the Invention
An aspect of the present invention relates to a new photochromic monomer, a photochromic polymer based on the same, a photochromic recording medium comprising the photochromic polymers, and a 3D read/write optical memory comprising the recording medium, more particularly to a new photochromic monomer for creating a two-photon photochromic recording medium for a 3D read/write optical memory with bit-by-bit recording of optical information, a photochromic polymers based on the same, a photochromic recording medium comprising the photochromic polymers, and a 3D read/write optical memory comprising the recording medium.
2. Description of the Related Art
Creation of data bases for telecommunication systems is a problem currently facing the information technology area. In this connection, the work is now in progress on developing an optical memory of ultra-high information capacity through the change-over from 2D information carriers to 3D recording media that allow the achievement of an information storage density as maximal as possible (up to 1 Tbit/cm3). For the 3D optical read/write memory to be created, two-photon recording media are required (I. Cokgor, F. B. McCormick, A. S. Dvornikov, M. Wang, N. Kim, K. Koblentz, S. C. Esener, P. M., Rentzepis. Proc. SPIE, vol. 3109, pp. 182-186, 1997; S. Kawata, Y. Kawata, Chem. Rev., vol. 100, pp. 1777-1791, 2000). Such media are being mainly developed in the USA (Call/Recall Corporation, Irvine and San Diego Universities of California) and Japan (Japan Science and Technology Corporation, Kyushu, Osaka, and Shizuoka Universities). Further, the use is made of polymer solutions of thermally irreversible photochromic compounds of different types: diaryl ethenes, fulgides, fulgimides, phenoxy derivatives of phenoxy naphthacene quinone and others. (A. S. Dvornikov, I. Cokgor, M. Wang, F. B. McCormick, S. C. Esener, P. M. Rentzepis. IEEE Transaction. Part A, vol. 20, N2, pp. 203-212, 1997). The compounds of this type are known to provide for the two-photon excitement and, consequently, the optical information recording in the medium volume.
The disadvantage of such recording media is that the photochromic layer is prepared from a monomer photochromic compound of one of the above listed types and a polymer binder. Such a medium essentially represents a molecular solution of the photochromic compound in a polymeric matrix. As a result, a concentration of photochromic molecules and, consequently, a number of photosensitive centers are determined by the ultimate solubility of the compounds in the polymer, that does not generally exceed 10% by the weight of dry polymeric binder. Crystallization processes of the photochromic substances, phase separation, aggregation of the photochromic molecules and formation of photochromic compound concentration gradients in the volume and on the surface of a layer are usually observed in such media (D. M. Buland, R. D. Miller, C. A. Walsh, Chem. Rev., vol. 94, p. 31, 1994). Thus, information capacity and data write/rewrite cyclicity in the photochromic medium are drastically reduced. Furthermore, a quantum yield for the photochromic transformations of some of the most acceptable photochromic compounds and, more particularly, of diaryl ethenes monomers, is not more than φ=0.5 due to the state of chaos in their distribution in the polymer volume (M. Irie. In: Organic Photochromic and Thermochromic Compounds. Eds. J. C. Crano and R. J. Guglielmetti. N. Y. and L., Plenum Press. 1999. V.1. P.207) and, consequently, a photosensitivity of the photochromic medium does not reach maximum values.
Due to this, of the most interest for the use as two-photon recording media are photochromic polymers comprising valence-bonded photochromic molecules either in the main polymeric chain or as side fragments. In the photochromic polymers, covalently bonded photochromic molecules are more stable in time during the storage owing to hampered migration thereof through the medium volume. The photochromic recording media include, for example, photochromic polymers having diaryl ethene fragments based on styrene and butyl methacrylate monomers (E. Kim, Y. K. Choi, M. H. Lee. Macromolecules, vol. 32, pp. 4855-4860, 1999; S. Y. Cho, H. W. Shin, K. H. Ahn, Y. R. Kim, E. Kim. Optical Materials, vol. 21, pp. 279-284, 2002). In comparison with the polymer solutions, the media of this type enable the formation of high quality layers having a thickness of 0.05-0.1 μm. The developed media allow the optical information recording when exposed to the radiation of a helium-cadmium laser (325 nm) and the erasing by using lasers emitting in the visible spectral range (663 and 532 nm). Nondestructive reading of the recorded information is achieved due to a photo-induced variation of the refractive index (Δn=0.0008) at the radiation wavelength of a semiconductor laser (830 nm).
Photochromic polymers based on 1,2-bis(3-thienyl)cyclopentene monomer (U.S. Patent Application Publication No. 2004/0030078 have been proposed. A disadvantage of such photochromic polymers is that they use a rather definite structure of the photochromic monomer and of the homopolymers based thereon. The disadvantage is caused by a limited choice of starting reagents for the synthesis of these photochromic polymers.